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Hydrothermal systems at oceanic spreading centers play an important role in the composition of seawater, the formation of ore deposits, the support of microbial and macrofaunal ecosystems, and even for the development of life on early earth. These circulation systems are driven by heat transport from the underlying magma chamber, where latent heat of crystallization and sensible heat from cooling are transferred by vigorous, high Rayleigh number convection through a thin conductive boundary layer. The traditional study of magmatic-hydrothermal systems is primarily based on the time-series observation, which takes the form of repeat visits, continuous offline monitoring by autonomous instruments, or continuous online monitoring by instruments with satellite or cable links to shore. Although a number of studies have deployed autonomous monitoring instruments at vents and around mid-ocean ridges to investigate geophysical and hydrothermal processes, the data are still rather limited and a comprehensive understanding of magma-hydrothermal processes at oceanic spreading centers is lacking. Numerical modeling needs to be employed to elucidate the dynamic behavior of magmatic hydrothermal systems and for testing completing hypotheses in these complex, data-poor environments. In this dissertation, I develop a mathematical framework for investigating heat transport from a vigorously convecting, crystallizing, cooling, and replenished magma chamber to an overlying hydrothermal system at an oceanic spreading center. The resulting equations are solved numerically using MATLAB. The simulations proceed step-by-step to investigate several different aspects of the system. First, I consider a hydrothermal system driven by convection, cooling and crystallization in a ~ 100 m thick basaltic magma sill representing an axial magma chamber (AMC) at an oceanic spreading center. I investigate two different crystallization scenarios, crystal-suspended and crystal-settling, and consider both un-replenished and replenished AMCs. In cases without magma replenishment, the simulation results for crystals-suspended models show that heat output and the hydrothermal temperature decrease rapidly and crystallinity reaches 60% in less than ten years. In crystals-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales. When magma replenishment is included, the magmatic heat flux approaches steady state on decadal timescales, while the magma body grows to double its original size. The rate of magma replenishment needed ranges between 5 x 10⁵ and 5 x 10⁶ m³/yr, which is somewhat faster than required for seafloor spreading, but less than fluxes to some terrestrial and subseafloor volcanoes on similar timescales. The heat output from a convecting, crystallizing, replenished magma body that is needed to drive observed high-temperature hydrothermal systems is consistent, with gabbro glacier models of crustal production at mid-ocean ridges. Secondly, I study the heat transfer model from a parametric perspective and examine the effects of both initial magma chamber thickness and magma replenishment rate on the hydrothermal heat output. The initial rate of convective heat transfer is independent of the initial sill thickness; but without magma replenishment, the rate of decay of the heat output varies linearly with thickness, resulting in short convective lifetimes and decaying hydrothermal temperatures for sills up to ~ 100m thick. When magma replenishment is included in crystals settling scenarios at constant or exponentially decreasing rates of ~ 10⁻⁸ m/s to the base of the sill, growth of the sill results in stabilized heat output and hydrothermal temperature on decadal timescales and a relatively constant to increasing thickness of the liquid layer. Sills initially ~ 10 m thick can grow, in principal, to ~ 10 times their initial size with stable heat output and a final melt thickness less than 100m. Seismic data provides evidence of AMC thickness, but it can not discriminate whether it denotes initial magma thickness or is a result of replenishment. These results suggest that magma replenishment might not be seismically detectable on decadal time scales. Periodic replenishment may also result in quasi-stable heat output, but the magnitude of the heat output may vary considerably in crystals suspended models at low frequencies; compared to crystals settling models. In these models the direct coupling between magmatic and hydrothermal heat output suggests that heat output fluctuations might be recorded in hydrothermal vents; but if damping effects of the basal conductive boundary layer and the upflow zone are taken into account, it seems unlikely that heat output fluctuations on a time scale of years would be recorded in hydrothermal vent temperatures or heat output. Thirdly, I extend the work to the binary system motivated by the fact that the real magmas are multi-component fluids. I focus on the extensively studied binary system, diopside-anorthite (Di-An), and investigate the effects of convection of a two-component magma system on the hydrothermal circulation system through the dynamic modeling of both temperature and heat output. I model the melt temperature and viscosity as a function of Di concentration, and incorporate these relations in the modeling of the heat flux. Simulations comparing the effects of different initial Di concentrations indicate that magmas with higher initial Di concentrations convect more vigorously, which results in faster heat transfer, more rapid removal of Di from the melt and growth of crystals on the floor. With magma replenishment, I assume that the magma chamber grows either horizontally or vertically. In either case magma replenishment at a constant rate of ~ 10⁻⁸ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and reasonable hydrothermal vent temperatures for decades. The final stabilized heat flux increases with increasing Di content of the added magma. Periodic replenishment with a 10 year period results in temperature perturbations within the magma that also increase as a function of increasing Di. With the simple magma model used here, one can not discern conclusively whether the decrease in magma temperature between the 1991/1992 and the 2005/2006 eruptions at EPR 9°50'N involved replenishment with more or less evolved magmas. Fourthly, I investigate a high-silica magma chamber as the hydrothermal circulation driver. I construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high Rayleigh number, well-mixed, crystallizing and replenished magma sill beneath a hydrothermal circulation system. Simulations comparing the time dependent heat flux from basalt, 0.1wt.% andesite, 3wt.% andesite, and 4wt.% dacite, indicate that higher viscosity magmas convect less vigorously, which results not only in lower heat transport and hydrothermal vent temperatures, but also in a lower decay rate of the vent temperature. Though somewhat colder, hydrothermal systems driven by unreplenished high-silica melts tend to have a longer lifetime than those driven by basalts, assuming a heat output cutoff of 10⁷ Watts. As in the basaltic case, magma replenishment at a rate of ~ 3 x 10⁵ - 3 x 10⁶ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and hydrothermal vent temperatures for decades. Idealized models of porous flow through the lower crust suggest such replenishment rates are not likely to occur, especially for high-viscosity magmas such as andesite and dacite. Long term stability of hydrothermal systems driven by these magmas requires an alternate means of magma replenishment. Finally, the dissertation concludes by discussing some avenues for future work. Most important of these are to: (1) couple magma convection with more realistic hydrothermal models and (2) link magma chamber processes to better physical models of replenishment and eruption.

Graduation date: 1997 / Best scan available for figures. Original figures are black and white photocopies.

Thesis (Ph. D.)--University of Washington, 2008. / Vita. Includes bibliographical references (leaves 131-141).

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 117-157).

Les connaissances des propriétés des fluides hydrothermaux, c'est-à-dire des solutions aqueuses à haute température (T) et haute pression (P), sont essentielles dans les domaines des sciences de la terre, chimie prébiotique, industrie nucléaire ou encore des sciences environnementales. Près du point critique (Pc, Tc) et dans les conditions pseudocritiques (i.e. à la densité critique au dessus de la Tc-Pc), les propriétés des fluides changent de manière radicale. Ces modifications ont lieu à différentes échelles: à l’échelle macroscopique (changements de densité et de compressibilité), à l’échelle mésoscopique (processus d’agglomération entre les différentes clusters) et enfin à l’échelle moléculaire (organisation local entre les atomes et les molécules, par exemple les liaisons hydrogène, H).Cette étude est centrée sur les effets de la solvatation aqueuse de différents électrolytes en fonction de la température, pression et concentration. L'étude expérimentale a été conduite par les mesures d’Absorption X, technique de choix pour mesurer d'une part l'évolution de la densité du fluide et d'autre part sonder l'environnement atomique local autour des solutés. Cette étude a été faite sur l'eau pure et sur différentes solutions salines à différentes pressions (jusqu'à ~1.3Pc) et à température variable (jusqu'à ~2Tc), pour passer du domaine liquide au domaine supercritique, dans une cellule permettant de découpler totalement l’effet de la pression et de la température.Nous avons obtenu des preuves expérimentales du déplacement du point critique et de l'isochore et de leur dépendance en fonction de la concentration en sel pour NaCl (0,3, 0,5 et 1,0 moles NaCl par kilogramme d'eau). Nous avons également observé une anomalie de densité dans la région supercritique (SC) et l'apparition d'une séparation des phases liquide-vapeur pour certaines solutions aqueuses. L'augmentation de la densité relative dans cette zone critique est plus prononcée pour Cs>Rb>K>Na>Li pour les bromures et les chlorures. Le changement structural dans cette région a été suivi par spectroscopie XANES haute résolution au seuil K du brome pour différents bromures d'alcalin. L'interprétation des spectres XANES indiquent clairement un changement drastique dans la structure locale du fluide au moment de l'apparition de cette anomalie pouvant être interprétée par l'apparition des paires ioniques.Ces nouvelles observations sont a relier au changement structurale du solvant et principalement à l'évolution de son pouvoir de solvatation lié à l'évolution des liaisons hydrogène à hautes T-P. / Knowledge of the properties of hydrothermal fluids, i.e. aqueous solutions at high temperature (T) and high pressure (P), are essential in the fields of earth sciences, prebiotic chemistry, nuclear industry and environmental sciences. Near the critical point (Pc, Tc) and under the pseudocritic conditions (i.e. at the critical density at T-P above Tc-Pc), the fluid properties change radically. These changes take place at different scales: the macroscopic scale (density and compressibility changes), the mesoscopic scale (agglomeration process between different clusters) and finally, the molecular scale (local organization of atoms and molecules, for example hydrogen bonds, H).This study focuses on the solvation effects in different electrolytes as a function of temperature, pressure and concentration. The experimental study was carried out by X-ray absorption measurements, the technique suitable for probing both fluid density evolution and local atomic environment around the solutes. This study was carried out on pure water and salt solutions at different pressures (up to ~ 1.3Pc) and temperatures (up to ~ 2Tc), to pass from the liquid to the supercritical domain, using a cell permitting to completely decouple the effects of pressure and temperature.We have obtained experimental proofs of the displacement of the critical point and the isochore and their dependence on the salt concentration in the case of NaCl (0.3, 0.5 and 1.0 moles NaCl per kilogram of water). We have also observed a density anomaly in the supercritical region (SC) and the appearance of liquid-vapor phase separation for some aqueous solutions. The relative density increase in the critical zone is more pronounced for Cs>Rb>K>Na>Li for bromides and chlorides. The structural change in this region was followed by high-resolution XANES spectroscopy at the K-threshold of bromine for various alkali bromide. The interpretation of the XANES spectra clearly indicates a drastic changes in the fluid structure related to this anomaly, which can be interpreted by the appearance of ionic pairs.These new observations are in link with water structure and solvation properties evaluation and consequently with hydrogen bonding changes under high T-P.

Hydrothermal vents are deep-sea hot springs. Vents are home to luxuriant assemblages of animals that colonize the warm venting fluids. High biomass is fed by microbes that use hydrogen sulphide and other reduced chemicals in the vent fluid as an energy source to fix inorganic carbon. Individual vents may persist for a few years to several decades. The specialized animals must find new vents, cope with changing fluid conditions and foster their offspring. The composition and structure of vent communities vary in space and time. My research at Axial Volcano, a seamount on the Juan de Fuca Ridge (JdFR) in the northeast Pacific, aims to find pattern in this variation and to propose viable hypotheses of the mechanisms driving the patterns. Axial is an ideal location as it supports mature vent fields (venting for over 15 years) and young, developing vents initiated by a volcanic eruption in 1998. Thus, I was able to study both temporal and spatial variation in vent communities at the same site and relate patterns of developing assemblages to patterns observed at longer-lived vents. Pattern detection is the first critical step in any community ecology study as it justifies and focuses the search for process. I have refined existing statistical methods and developed novel techniques to test for pattern in vent species distributions and abundances. I modified an existing null model approach and showed that species distributions among sixteen vents differ from random in a long-lived (>15 years) vent field. I also developed a novel null model to confirm that initial patterns of community assembly seven months following the Axial eruption differ from random recruitment of species and individuals to new vents. My description of the community response to the Axial eruption is the first quantitative report of patterns of vent colonization and succession. My work documents that new vents are colonized quickly (within months) and that initial assemblages are variable. However, rapid community transitions and species replacements within the first few years cause new assemblages to resemble mature vents by 2.5 years post-eruption. Three habitat factors correlate with the development of nascent vent assemblages: the recruitment timing of the tubeworm Ridgeia piscesae post-eruption, vent age and vent fluid hydrogen sulphide content. I also describe a new polynoid polychaete discovered colonizing the new vents in high densities. My major contribution to vent community ecology is revealing species patterns through extensive sampling and rigorous statistical methods. These patterns are a necessary step towards understanding the processes that structure vent communities: they direct future research effort towards the key species and generate hypotheses to be experimentally tested. My work also elucidates how vent species respond to habitat destruction and creation, which is critical information for effectively managing Canada's only hydrothermal vent Marine Protected Area on the JdFR. / Graduate

This work seeks to explore current ecological theory through application to communities inhabiting hydrothermal vents. This thesis aims to: (1) add to and synthesise knowledge of species and their distributions at the intra- and intersegment scale; and (2) evaluate vent community patterns and speculate on processes. Samples used are submersible grabs of low temperature (<60°C) tubeworm assemblages on basalt and sulphide surfaces. Species abundances and distributions on three segments of the Juan de Fuca Ridge (Axial, Cleft, and CoAxial) are described. Community descriptors such as species density, Simpson's and the Shannon-Wiener diversity indices, evenness, species richness, species abundance-distribution models, species percent-average relative abundance and density are used. Vent community structure is compared among segments using these descriptors, visual descriptions, pairwise correlations, Friedman tests of distributions, cluster and correspondence analysis, rarefaction, complementarity, a test for saturation, and Whittaker's beta diversity. Vent community composition on Axial, north Cleft, and CoAxial is similar at the segment and inter-segment scale. The limpet Lepetodrilus fucensis is the most abundant species at all sites. Differences among communities are best seen temporally, not spatially. Senescent communities can be distinguished from active vent assemblages. Pioneer communities, however, are statistically indistinguishable from intermediate communities when sampled two or more years post-eruption. Axial and Cleft species dispersion fits the core-satellite hypothesis. The exceptions are the polynoids Branchinotogluma sp., Lepidonotopodium piscesae, and Levensteiniella kincaidi, which are widespread and present in low local abundances. Both local and mesoscale regional mechanisms explain observed local diversity. Spatial isolation, not habitat differences, influences between-habitat diversity (beta diversity) on Axial, Cleft, and all three segments combined. Meiofauna are important for species richness estimates, identifying differences among structurally similar communities, and understanding input/output between vents and the deep-sea. Measurements such as species richness and diversity indices may be poor at distinguishing among vent communities because vents are species poor and uneven. The Michaelis-Menten, Jackknife 2, and Chao 2 nonparametric vent species richness estimators perform well with small samples. Vent communities should be compared to habitats of similar diversity and evenness as well as disturbance and productivity regimes. Candidate comparison communities include communities in early successional states, selected taxocenes such as carabid beetles on fungi, or high disturbance and/or low diversity systems like the rocky intertidal, organically polluted sediments and oxygen minimum zones below upwelling regions in the deep-sea. / Graduate

Critical properties of the NaCI-H₂0 fluid system are of fundamental interest to a variety of geochemical applications including fluid inclusion studies, numerical modeling of hydrothermal systems, and development of theoretical models for two·component fluid systems. Although many workers have expressed interest in NaCl·H₂0 fluid critical properties, most studies have been limited to small compositional ranges with little agreement among data sets at higher salinities. Critical densities are recorded in only one of these reports, and no studies have determined the locations of NaCl-H₂0 critical isochores (PT projections of critical densities). Furthermore, no studies to date have determined critical properties of NaCl·H₂0 solutions in excess of room temperature saturation (26.4 wt.% NaCl). / Master of Science